1. Field of the Invention
The present invention relates to low loss glass-ceramic materials and methods for preparation of same. More particularly, the present invention relates to compositions and methods for preparation of low CTE glass-ceramic based materials which allow the dielectric constant of the materials to be varied to meet the electrical performance requirements of chip carriers or electronic devices.
2. Background and Related Art
In the fabrication of digital electronic devices, a combination of design factors, such as, metallization line height, width and conductivity and ceramic layer thickness and dielectric constant K, are chosen to maximize both signal transmission speed and metallization and I/O density while at the same time maintaining the desired impedance which is typically that of the various devices and packaging components utilized in assembly of the devices. As digital electronic device applications move into the high frequency marketplace, need will arise for higher dielectric constant ceramic materials that can be used for integrating capacitors within the electronic package, as well as allow metal features to be made smaller for miniaturization of, for example, active filters that can be built into the ceramic. For RF/analog applications, different dielectric constants may be desirable (as well as varied metal line widths/heights) in order to build inductance, capacitance and resonant filter structures that not only have the desired properties, but also take up minimum space to thereby reduce device volume. Often, both digital and analog functions are combined within the same ceramic substrate.
In addition to varying dimensional characteristics in the design of electronic devices, it is also desirable to vary material parameters and, at times, it may also be desirable to vary both dimensional characteristics and material parameters. Typically, varying material parameters involves changes in ceramic composition. Changes in ceramic composition adds cost to manufacturing by requiring multiple raw materials formulations be readily available. In addition, this approach limits the ranges of dielectric constants, for example, to those produced by individual glass, glass-ceramic or ceramic compositions. The range of compositions that may be used is also limited to those having sintering characteristics compatible with those of the electronic package/substrate of which it is a part. Moreover, manufacturing limitations may make it preferable to select from certain ranges of metal line dimensions and ceramic layer thicknesses to produce the highest manufacturing yield and lowest cost.
For the above-described applications and, in particular, high frequency applications it is desirable that the dielectric loss tangent or dissipation factor be kept to a minimum. In addition, it is also desirable to be able to increase the dielectric constant of working glass-ceramic (GC) formulations without significantly affecting electrical resistivity, CTE, fired density or microstructure.
It is known in the art to use molybdenum, tungsten and their oxides as a way to increase the dielectric constant of alumina-based glass bonded ceramics, when these are fired in suitably reducing conditions. However, molybdenum will not survive the steam ambient typically used for binder removal in low temperature glass-ceramic sintering processes. In addition, molybdenum and tungsten cannot be used for air sintering.
U.S. Pat. No. 4,870,539 to Chance et al describes a high dielectric constant, low dielectric loss tangent glass-ceramic material for use in fabricating GBBL capacitors. Chance et al describe making these titanium containing capacitors by crystallizing glass-ceramic compositions formulated to develop cordierite with either BaTiO3 or SrTiO3 in order to form multiphase materials that achieve dielectric constants of approximately 200 to 300 at 10 MHz. These are specially formulated compositions that are completely vitreous or glassy (crystalline free) before the final sintering/densification heat treatment.
In accordance with the teachings of the present invention, a composition and method of preparation of low CTE glass-ceramic based materials is provided which allow the dielectric constant of the composition to be selectively increased to meet the electrical performance requirements of chip carriers or electronic devices. A cordierite-based glass-ceramic is the preferred starting composition to which additives are combined to selectively increase the dielectric constant. Other glass-ceramic formulations, such as those based upon beta-spodumene, mixtures of glass and ceramic (such as cordierite, mullite, alumina, or the like), or other low CTE materials, may also serve as the main component of the mixture.
The additives are selected so that, in addition to increasing the dielectric constant, they do not significantly impact the low thermal expansion or the sintering shrinkage kinetics of the glass-ceramic, essentially maintain or reduce the loss tangent or dissipation factor of the glass-ceramic and are compatible with existing electronic packaging fabrication requirements and conditions.
Accordingly, it is an object of the present invention to provide an improved glass-ceramic composition and method for making same.
It is another object of the present invention to provide a low loss tangent glass-ceramic composition with modifiable dielectric constant.
It is a further object of the present invention to provide a low loss tangent glass-ceramic with modifiable dielectric constant which exhibits a low CTE.
It is yet a further object of the present invention to provide a low loss tangent glass-ceramic with modifiable dielectric constant which is compatible with low sintering temperature metallizations and which can be fired with such metals in any one of a reducing, oxidizing or neutral atmosphere or all, as required.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention.